Spring-summer net community production, new production, particle export and related water column biogeochemical processes in the marginal sea ice zone of the Western Antarctic Peninsula 2012-2014

Abstract

New production (New P, the rate of net primary production (NPP) supported by exogenously supplied limiting nutrients) and net community production (NCP, gross primary production not consumed by community respiration) are closely related but mechanistically distinct processes. They set the carbon balance in the upper ocean and define an upper limit for export from the system. The relationships, relative magnitudes and variability of New P (from ¹⁵NO₃^- uptake), O₂ : argon-based NCP and sinking particle export (based on the ²³⁸U : ²³⁴Th disequilibrium) are increasingly well documented but still not clearly understood. This is especially true in remote regions such as polar marginal ice zones. Here we present a 3-year dataset of simultaneous measurements made at approximately 50 stations along the Western Antarctic Peninsula (WAP) continental shelf in midsummer (January) 2012-2014. Net seasonal-scale changes in water column inventories (0-150 m) of nitrate and iodide were also estimated at the same stations. The average daily rates based on inventory changes exceeded the shorter-term rate measurements. A major uncertainty in the relative magnitude of the inventory estimates is specifying the start of the growing season following sea-ice retreat. New P and NCP(O₂) did not differ significantly. New P and NCP(O₂) were significantly greater than sinking particle export from thorium-234. We suggest this is a persistent and systematic imbalance and that other processes such as vertical mixing and advection of suspended particles are important export pathways.This article is part of the theme issue 'The marine system of the west Antarctic Peninsula: status and strategy for progress in a region of rapid change'.

Keywords: Antarctica; carbon export; net community production; ocean biogeochemistry; thorium-234 deficiency.

Figure 1.

Surface ocean metabolic processes driving the balance between gross primary production and respiration. This balance is the net community production or NCP. GPP and NPP are gross and net primary production, respectively, and R is community respiration by autotrophic, heterotrophic and mixotrophic plankton. Dissolved organic matter (DOM) is produced by phytoplankton and consumers and respired by heterotrophic bacteria. Exogenous (new) nutrients (e.g. nitrogen and iron) support a varying fraction of the GPP (new production) and can be supplied or removed by both vertical and horizontal processes. The new production and NCP are available for export from the system. Atmospheric exchanges are additional source and sink terms for nutrients and organic matter supporting new production and/or export. The figure is not meant to convey the exact vertical distribution of the processes. (Modified from Ducklow & Doney [1]). (Online version in colour.)

Figure 2.

Vertical profiles of nitrate, iodide and the thorium-234 deficiency at the four corners of the sampling region in January 2012. (a,b) Northern stations 600.040 and 600.200. (c,d) Southern stations 200.040 and 200.200. The 0.040 (a,c) stations are coastal and the 0.200 stations are deep (greater than 3000 m) offshore stations. (Online version in colour.)

Figure 3.

Scatterplots of discrete depth [NO₂ + NO₃], iodide concentrations and thorium-234 deficiencies relative to uranium-238 in January 2012. The iodide–nitrate relationship is discussed in the text. (Online version in colour.)

Figure 4.

(a–i) Distribution of rates of export, new production (nitrate uptake) and NCP(O₂) along the WAP in January, 2012–2014. (Online version in colour.)

Figure 5.

Box plots of production and export rates along the WAP shelf in January 2012. Each box represents the water column integrated data from all the stations as given in electronic supplementary material, table S3. (NPP (¹⁴C): rates of daily net primary production; ΔNO₃: average daily rates of NO₃ removal for the specified ice retreat criteria; ΔNO₃–I: NO₃ removal converted from iodide inventory accumulation; export (²³⁴Th): daily export rate from ²³⁴Th; New Prod: daily rate from ¹⁵NO₃ incorporation; NCP(O₂): daily rate of NCP from mixed layer O₂ budget as measured by EIMS. The top and bottom of the boxes show the 75% and 25% percentiles, respectively. The capped vertical lines indicate the 90th and 10th percentiles and symbols indicate outliers. The line inside the box is the median. Rates sharing letters are not statistically different averaged over the sampling region within each year (T-test, p > 0.05). Rates with different letters are significantly different (p < 0.05). All values were converted to nitrogen equivalents as described in Methods and materials. (Online version in colour.)

Figure 6.

Box plots of production and export rates along the WAP shelf in January 2013 as in figure 5 with data from electronic supplementary material, table S4. Iodide was not measured in 2013. (Online version in colour.)

Figure 7.

Box plots of production and export rates along the WAP shelf in January 2014 as in figure 6, with data from electronic supplementary material, table S5. Iodide was not measured in 2014. (Online version in colour.)